1. Field of the Invention
This invention relates to an optical recording and/or reproducing apparatus wherein an optical tape, an optical disc or the like is used as a record medium, and more particularly to an optical recording and/or reproducing apparatus of the type mentioned wherein a light beam is scanned using an acousto-optic deflector (AOD).
2. Description of the Prior Art
Optical recording and/or reproducing apparatus have been conventionally proposed wherein an optical tape, an optical disc or the like is used as a record medium and a light beam is scanned across such record medium by means of an acousto-optic deflector (AOD) to effect recording or reproduction of information signals on the record medium. One of such optical recording and/or recording apparatus is disclosed, for example, in Japanese Patent Application No. 61-261724 which has been filed in advance before the Japanese Patent Office by the applicant of the present patent application.
Such an optical recording and/or reproducing apparatus as described just above includes, as shown in FIG. 1, a laser beam source 101 which may be, for example, a laser diode. A light beam emitted from the laser beam source 101 is modulated, in the recording mode of the apparatus, in response to information signals to be recorded so that the intensity of the light beam is varied, but in the reproducing mode, it has a fixed intensity lower than the intensities of the light beam in the recording mode. The light beam is then transformed into a parallel laser beam by a predetermined optical part such as a collimator lens 102 and then introduced to an acousto-optic deflector 103. The incident light beam to the acousto-optic deflector 103 is controlled to be deflected by the acousto-optic deflector 103. The acousto-optic deflector 103 has a transducer 103a and an absorbing element 103b provided at a pair of opposing side faces thereof. The acousto-optic deflector 103 receives, at the transducer 103a thereof, a driving signal delivered from a deflector driving circuit 103c and repeatedly changes the deflection angle of the light beam within a predetermined angular range of the direction indicated by an arrow mark .alpha. in FIG. 1.
The light beam then passes a cylindrical lens 104 and a pair of relay lenses 105 and 106 and comes to an objective lens 107. The objective lens 107 focuses the light beam on a record face 109 of an optical record medium 108.
The light beam incident to the acousto-optic deflector 103 is controlled to deflect within the predetermined angle of the direction indicated by the arrow mark .alpha. in FIG. 1 so that it may be scanned such that the position on the record face 109 to which the light beam is focused by the objective lens 107 may be within a range from a first focal point indicated at f.sub.0 in FIG. 1 to a second focal point indicated at f.sub.1 in FIG. 1.
Meanwhile, the optical record medium 108 is moved in a direction perpendicular to the plane of FIG. 1, that is, in a direction perpendicular to the scanning direction of the light beam.
In the recording mode of the apparatus, the record face 109 of the optical record medium 108 undergoes chemical or physical changes by irradiation of a light beam, thereby effecting recording of information signals.
In order to reproduce, on the optical recording and/or reproducing apparatus, the information signals thus recorded on the optical record medium, a light beam reflected from the record medium 109 is taken out for detection thereof, for example, by means of a beam splitter 110 interposed between the collimator lens 102 and the acousto-optic deflector 103 as disclosed in Japanese Patent Application No. 61-261724 which has been filed in advance before the Japanese Patent Office by the applicant of the present patent application. In particular, in the reproducing mode of the apparatus, the light beam is scanned in such a manner as described above, reflected by the record face 109 and introduced again to the objective lens 107. Then, the reflected light beam passes the relay lenses 106 and 105, cylindrical lens 104 and acousto-optic deflector 103 and is introduced to the beam splitter 110. The reflected light beam is then reflected by the beam splitter 110 and introduced to an optical detecting means 111 which may comprise a detecting lens, an optical detector or the like. The optical detecting means 111 thus detects the reflected light beam from the beam splitter 110 to effect reproduction of the information signals.
By the way, in the case of the acousto-optic detector 103 which is used with such an optical recording and/or reproducing apparatus as described above, the light beam incident thereto is separated into a zero order light beam which passes the acousto-optic deflector 103 without causing diffraction and a first order light beam which passes the acousto-optic deflector 103 while being diffracted by the latter. The zero order light beam after having passed the acousto-optic deflector 103 is a light beam which is equivalent in its advancing direction and polarized condition to the incident light beam. Meanwhile, the first order light beam is controlled to deflect and also changed in polarized condition by the acousto-optic deflector. Accordingly, the first order light beam after having passed the acousto-optic deflector 103 is a light beam which is in a linearly polarized condition in the direction perpendicular to the direction of polarization of the incident light beam.
The zero order light beam then comes to the record medium 109 similarly to the first order light beam. Since the zero order light beam is not controlled to deflect by the acousto-optic deflector 103, it is always focused at a position on the record face 109 of the optical record medium 108 a little displaced from the scanning range of the first order light beam from the point f.sub.0 to the other point f.sub.1 in FIG. 1.
In this instance, the zero order light beam focused on the record face 109 of the optical record medium 108 writes, in the recording mode of the apparatus, information signals on the record face 109 similarly to the first order light beam. On the other hand, in the reproducing mode, the zero order light beam is reflected by the record face 109 similarly to the first order light beam. The zero order light beam reflected by the record face 109 then comes to the optical detecting means 111 similarly to the first order light beam reflected by the record face 109.
As the zero order light beam is focused on and reflected by the record face 109 and then comes to the optical detecting means 111, accurate recording or reproduction is not assured.
It may be recommended, in order to prevent such a bad effect of the zero order light beam which reaches the record face 109 as described above, to provide a screen plate 112 at an intermediate position between the relay lenses 105 and 106 at which the zero order light beam is focused as shown in FIG. 1. With the arrangement, the zero order light beam is interrupted by the screen plate 112.
However, when the first order light beam approaches the zero order light beam, that is, when the angle at which the first order light beam is diffracted by the acousto-optic deflector 103 is small, there is the possibility that the first order light beam may be interrupted together with the zero order light beam by the screen plate 112. Accordingly, recording or reproduction of information signals by the first order light beam is permitted only when the first order light beam is scanned within an angular range of the scanning angle thereof in which it is not interrupted by the screen plate 112.
The angular range within which the first order light beam is controlled to deflect by the acousto-optic deflector 103 is several degrees and thus small. Within the angular range within which the first order light beam is controlled to deflect, the angular range effective for recording and/or reproduction of information signals is further reduced by the screen plate 112. This has a bad influence on improvements in recording density of information signals on the record face 109 of the optical record medium 108. Besides, it is required to maintain the location of the screen plate 112 with a high degree of accuracy, which complicates assembly and adjustment of the apparatus and thus deteriorates practical use.
Otherwise, it may be recommended to separate the zero order light beam and the first order light beam from each other using an optical part wherein the transmissivity has a polarization dependency such as, for example, a polar screen making use of a difference in polarized condition between the zero order light beam and the first order light beam. It is necessary, however, to provide means for separating the zero order light beam and the first order light beam from each other and independent means for taking out the light beam reflected by the record face 109 of the optical record medium 108 to detect the light beam. This results in a complicated construction of the apparatus.